1. Field of the Invention
The present invention relates to image processing, and in particular to an image compensation method collocating with an auto white balance (AWB) function.
2. Description of the Related Art
Generally, an image extraction device extracts an image using a charge coupled device (CCD) and obtains red, green, and blue (RGB) signals corresponding to the extracted image. Color shift of the RGB signals for an object may be detected due to different color temperatures of the ambient light. An extracted image for a white object, for example, represents a color shift towards red and yellow when a color temperature of the ambient light is low, and, toward to cyan and blue when a color temperature of the ambient light is high.
Currently, an image extraction device (a digital camera, for example) provides auto focus (AF), auto exposure (AE), and AWB to yield an extracted image approaching the real object. AWB modifies the light and shade of an image for color compensation. Colors for an object may be changed due to projected light. The human eye can automatically correct color variation but a camera lens does not provide such function. Thus, AWB enables a camera to achieve color variation, presenting real white based on different conditions by calculating an illumination average and automatically adjusting color balance. Alternatively, AWB calculates illumination according to image attributes and adjusts color degrees of red, green, and blue to correct color bias.
A white balance correction method disclosed converts RGB coordinates of pixels of an extracted image to YCbCr coordinates and calculates averages of Y, Cb, and Cr signals of each sampling blocks. Cb and Cr act as two axes, four quadrants are created according to the two axes, and adjustment is implemented based on the direction shifting to one of the four quadrants. The described AWB correction method can be more accurate if more calculation conditions are obtained, but this requires complicated calculations, which are time-consuming.
Other AWB correction methods comprise a gray-world assumption method and a method for searching light spots. The gray-world assumption method considers the color of the entire extracted image as neutral gray. When a color of the image does not belong to the neutral gray, a R gain value and a B gain value is adjusted to approach the difference there between RG and BG to zero and allow the current gain value to act as the neutral gray. The method, however, cannot perform white balance well such as using the image extraction device to extract an image as the view of the forest or the view of the sunset. The method for searching light spots considers high occurrence probability of the white color around the location of light reflection. The method is implemented by determining whether characteristics of a light spot are precise but frequently results in a wrong determination.
Thus, based on the drawbacks of disclosed white balance methods, an improved image compensation method is desirable.